1. Field of the Invention
The present invention relates generally to an improvement in an optical amplifier such as an erbium doped fiber amplifier (EDFA), and more particularly to an optical amplifier suitable for broad-band wavelength division multiplexing (WDM) and a system having the optical amplifier.
2. Description of the Related Art
In recent years, a manufacturing technique and utilizing technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying an optical signal or signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and a pumping unit for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has been developed to amplify signal light having a wavelength band of 1.55 xcexcm where the loss in a silica fiber is minimum. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 xcexcm band or a 1.48 xcexcm band, a gain band including a wavelength band of 1.55 xcexcm can be obtained.
Further, another optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain main signal light (WDM signal light), which is output to an optical fiber transmission line. At a receiving end, the main signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data (a main signal) is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity by a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain of the optical amplifier which characteristic is represented by a gain deviation or gain tilt. For example, in a typical EDFA, it is known that a gain deviation is produced at wavelengths near 1.55 xcexcm. If the gain deviations in a plurality of cascaded EDFAs are accumulated, an optical SNR (signal-to-noise ratio) in a channel included in a low-gain band is degraded. Accordingly, to allow high-quality transmission, it is preferable to flatten the wavelength characteristic of gain of an optical amplifier.
In a conventional EDF, an EDF so designed as to obtain a flat wavelength characteristic of gain in a short-wavelength band or conventional wavelength band (referred to as xe2x80x9c1.55 xcexcm bandxe2x80x9d or xe2x80x9cC band: center band or conventional bandxe2x80x9d) defined by the range of about 1.53 to 1.57 xcexcm is used as an optical amplifying medium. However, the width of a wavelength region where the wavelength characteristic of gain in the C band is flat is about 15 to 20 nm. Accordingly, in the case that the wavelength spacing is set to 0.8 nm, the number of channels of WDM signal light becomes about 25. In the case that the wavelength spacing is set to 0.4 nm, the number of channels of WDM signal light becomes about 50. In general, the wavelength spacing is limited by the bit rate of each signal, techniques of optical multiplexing/demultiplexing in a terminal device, nonlinear effects in an optical fiber transmission line, etc.
To further increase the number of WDM channels, research and development are being pursued on the use of a long-wavelength band (referred to as xe2x80x9c1.58 xcexcm bandxe2x80x9d or xe2x80x9cL band: long wavelength bandxe2x80x9d) defined by the range of about 1.57 to 1.61 xcexcm. In an EDF for the L band, the width of a wavelength region where a flat wavelength characteristic of gain is obtained is about 30 nm. Thus, the EDF for the L band can broaden the band where a flat wavelength characteristic of gain is obtained as compared with the EDF for the C band. Further, by combining an optical amplifier for the C band and an optical amplifier for the L band to configure an optical amplifier, the band where a flat wavelength characteristic of gain is obtained can be further broadened.
Although the above-mentioned optical amplifier configured by the combination of the C band and the L band has an advantage on band broadening, optical components or the like for constructing each of the optical amplifiers for the C band and the L band are required, causing a complicated configuration of the optical amplifier as a whole.
It is therefore an object of the present invention to provide an optical amplifier having a broad band and a simple configuration. It is another object of the present invention to provide a system having such an optical amplifier. Other objects of the present invention will become apparent from the following description.
In accordance with an aspect of the present invention, there is provided an optical amplifier comprising first and second optical amplifiers, an optical demultiplexer, an optical multiplexer, and a control unit. The first optical amplifier provides a gain for a first band. The second optical amplifier provides a gain for a second band different from the first band. The optical demultiplexer optically couples the first optical amplifier and an input port by the first band, and optically couples the second optical amplifier and the input port by the second band. The optical multiplexer optically couples the first optical amplifier and an output port by the first band, and optically couples the second optical amplifier and the output port by the second band. The control unit controls the first and second optical amplifiers so that the optical power at the output port becomes constant.
With this configuration, the optical demultiplexer and the optical multiplexer respectively have a wavelength demultiplexing function and a wavelength multiplexing function, so that an optical signal supplied to the input port and having a wavelength included in the first band is amplified by the first optical amplifier and the amplified optical signal is output from the output port, whereas another optical signal supplied to the input port and having a wavelength included in the second band is amplified by the second optical amplifier and the amplified optical signal is output from the output port. Accordingly, an optical amplifier having a broad band can be provided. Further, the control unit operates commonly to the first and second optical amplifiers so as to maintain the optical power at the output port constant. Accordingly, a circuit for ALC (automatic level control) can be formed commonly for the first and second optical amplifiers, thereby simplifying the configuration of the optical amplifier.
Each of the first and second optical amplifiers may comprise a doped fiber doped with a rare earth element, and a pumping source for supplying pump light to the doped fiber. In this case, the control unit may control the powers of the pump lights to be supplied to the doped fibers of the first and second optical amplifiers.
In the case that an EDF is used as the doped fiber, the first band may include a C band defined by the range of 1.53 to 1.57 xcexcm, and the second band may include an L band defined by the range of 1.57 to 1.61 xcexcm. In the case that the doped fibers of the first and second optical amplifiers have the same composition, the doped fiber of the second optical amplifier may be set longer than the doped fiber of the first optical amplifier, thereby making the first and second optical amplifiers provide gains for the C band and the L band, respectively.
In accordance with another aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting WDM signal light obtained by wavelength division multiplexing at least one optical signal having a wavelength included in a first band and at least one optical signal having a wavelength included in a second band different from the first band; and at least one optical repeater arranged along the optical fiber transmission line. Each of the at least one optical repeater includes the optical amplifier according to the present invention.
This system employs the optical amplifier according to the present invention, so that the transmission of large-capacity data can be performed by broad-band wavelength division multiplexing, and the configuration of the system can be simplified.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.